Partially dehydrated chrysotile fiber and method of making



" M. s. BADOLLET ET AL FARTIALLY DEHYDRATED CHRYSOTILE FIBER AND METHOD OF MAKING Filed Sept. 20, 1947 Nov. y4r, 1952 IN V EN TORS C'. .frei/.5.

Patented Nov. 4, 1952 ngporationoffN ew York Application september 2o, 1947; 'serial 775310 andi 'ifened fibers sarees ath rlongitddinany transversely into nner: nfbras'la gregatese ishorteriength. Durmgsuenepenngfandzrnning operations they ends ofitnercrysta aggregates :are :shatteredor Vfrayed `to nfdevelop :ai great `Lnunfib'e' of Srninute hairl'ike fibrous filaments; YV'Iln'esev Afiberizing1treatmt'erits .imbroverth elting Ycharacteristics'of the be'rs and:therreb improve the strengthof"sheetsproe duced-shy fthef Wet.l process sheetingy operation. A .primaryvebject :oftlie @present invention fis to so treatj the fibers 1 las to further irnprove them :for manufacture izo'f zwater-*laid z'slieets rand @shapes .,Onevofttne .fchi'ef difficulties 'which is encountei-ed in `athe formation :fof wateralaid asbestos :fberssheets isfin' thee-comparative: slowness iwith rwhich fw'aterz separates .from tlfie1 sheet fby f filtra- `tion fas :the-:sheet is being `Vformed.. The `slow -1ltering;rate offsuchrwaterelaidielted'ibers rn'a- `terialljy freduces @the speed 'of t:operation of *the .sheetiformingiprocesa y Y Ai-particular object fis-to provide method of treatinga'sbesto's fibers to mproveftlielr freeness or ltering proprties'tobetter adapt 'them for 'a wet 'lprcessfshet ffo'rrin'g operation.

'Various methods have been heretofore ro- 'posed ffi treating asbestos o'is'to Vililpdve `S`11Ch 'bers'fo'r a particular' purpose. In general such methods havelhada-'s'erious adverse aifect on` the strength or feltingcharacteristics of the fibers.

2i-ny'treatment ofthe brs 'which'tends to irnpa't' br'ttleness thereto, orf-'Which increases 'their' tendency to `form agglomeraties 'or flocks in water "suspension, has anffadverse effect bnftliluse fof thebe'rs4 for -we-t Y"process `sheet inanufacture by `reducing*the possi-nity of forming' a uniformly 'flted *ber `:sheet of `suitable strength yand- 'of uniform iine textre.

Another object is to provide a /rneth'od "f `increasing K the 'free Afiltering characteristics `oi? jasbstos berswithout:substantial adverse effect (inther tensile strength and felting characteristics (f't'he bIS.

With" the vabove'objects in viewthe invention .consists in the improved method of treating as- -bestos bers to impart freeness'thereto which is l'feilijft des d and'mr'e `15ar1'ic1'11`a`r13""d e- `f a'nynlg'clairhs.

ba'iing sdiltri'frece 4will miriade 'totne "attaehed drawiarin which the fsi'nleui'e fportr'a'ysain vertical section, vwith .parts inelvatio, a shaft furnace-Which maybe .used for. flash hatng chrysotile b"as-bestos "bers in.acccrdancev iii/ith'tl'is invention. .l

A AAsbestos brs of the chrysot'ile type which .givevthe most difficulty in paper andsheet formoperations becauseyof slow filtering characteristics,` develop V-aslimyfsuriace coating when Wet. .The invention is -basedon the discovery that suchrbers of slimy and slow filtering characteristicscan beconverted by'a'moderate and short i -heat treatment into iibers of comparatively much .improvedV filtering characteristics yhaving no duction of tensile strength and felting properties of the fibers.

The invention has general application t the treatment of chrysotile asbestos fibers to enhance their filtering characteristics for use in the manufacture of water-laid felted fiber sheet products. The process has particular utility for treating the shorter commercial grades of chrysotile asbestos fibers ranging as to size classification over groups 3-7, including fioats, in accordance with the Standard Quebec Screen Classification Scale.

The process consists essentially of subjecting the fibers to brief heat treatment whereby the fibers are heated to a temperature in the range 600-l200 F. and are then cooled after being heated at such temperature for a period not exceeding three minutes. By careful control of the temperature and time of heat treatment the filterability of the fibers can be greatly improved without serious adverse effect on the strength and felting properties of the fiber.

The freeness or filterability of chrysotile asbestos fibers is improved by raising the fiber surfaces to temperatures in the range between 600- 700o F. and 900 F., and maintaining the fiber/at such temperatures for about three minutes. As little as five to ten seconds expo-sure of a chrysotile fiber to direct contact with hot gases at gas temperatures in the range of 12001500 F. has the effect of markedly reducing the water of crystallization at the fiber surfaces and improving the filtering characteristics. Any heating of the fibers at a fiber temperature above 1200 F. for more than three minutes has a serious adverse effect on the tensile strength and felting properties of the fiber.

One method of carrying out the heat treatment is by loading the loose fibers at room temperature in a layer of 1 3 inch thickness on a conveyor belt, and transporting the fibers at a controlled rate through the combustion chamber section of a drying oven in which the gas temperatures are maintained within the range of about 900-1500 F. The fibers are heated within the chamber for a short period, and are then removed from the chamber for quick cooling to avoid the harmful effects of prolonged exposure to the high temperature gases. This particular method of heat treating the fibers is not the preferred method because of non-uniformity of heat transfer between the heating gases and fibers throughout the full depth of the bed of fibers.

It is difficult to effect uniform controlled heatlng of asbestos fibers by exposing them in a layer or deep bed to hot gases flowing over or around the bed. Some time is required to preheat the fibers from room temperature up to the temperature of the hot gases. Furthermore, the rate of heat transfer between hot gases and fiber surfaces varies with the temperature differential between the gases and the fibers, and with the relative rates of movement of gas and fibers, and is much more rapid at the surface of a deep layer of` fibers than it is in the interior of the layer.

According to another heating method a body of commercial chrysotile fibers is introduced through a fiuff box onto the surface of an endless conveyor, while advancing the conveyor at a rate controlled to build up a bed of the fibers several inches thick. The fibers are well opened and fiuffed during their passage through the fiuff box, and the layer of fibers on the conveyor is compressed by a press roll to a mat. The mat of fibers thus-formed is transferred to the surface of an endless 'asbestos fabric conveyor which carries the fibers through a dielectric heating field between electrodes of a radio frequency electromagnetic heater. Using a heater having electrodes with a heating area of 2 ft. x 6 ft., and with the conveyor travelling at a speed of 1.65 ft. per minute, a bed of fibers weighing 2O lbs. was exposed to the heating field for a period of about 3.5 minutes. This method of heating rapidly raises the fibers throughout the bed from a temperature of 50 F. to 600-900 F. Immediately after the fibers have been exposed to the electro-magnetic eld for not to substantially exceed three minutes at the indicated temperature, the fibers are transferred from the asbestos conveyor to a pneumatic conveyor by means of which the heat treated fibers are transported to a collector and simultaneously cooled by the air which serves as the carrier medium.

The freeness or filterability of fibers thus heated was improved as much as -95%, while the water of crystallization was reduced only about 10% from an original average value of about 131/2%, to 12-121/2 The tensile strength of the fibers was not lowered more than 25% bythe heat treatment. The heat treatment is usually applied to fibers containing not to exceed about 2% by weight of uncombined or surface moisture. The heating operation is controlled as to time and temperature to eect a large increase in fiber freeness while avoiding substantial loss of fiber strength and felting properties.

A very satisfactory method of carrying out the heat treatment is to project or drop the fibers in loose form into a flowing stream of hot gas, in such a way as to expose each individual fiber to direct hot gas contact for a short heating period, followed by removal of the fibers from the heating gas stream, and rapid cooling. This method is easy to control to secure uniformity of heat treatment, because it is possible to develop a substantially uniform period of contact of each fiber with the hot gas, and to provide for a substantially uniform rate of heat transfer between the hot gas and the fibers.

y As shown in the attached drawing, chrysotile bers can be continuously fed at a controlled rate by feeder belt I0 and chute I2 into a carrier stream of recycled hot Acombustion gases which impart some degree of preheat to the fibers, after which the gas suspended fibers enter a cyclone separator I4 and are projected downwardly therefrom in gas suspension intoy and through a rising stream of hot combustion gases in a vertical shaft I6 having smooth walls. The fibers fall by gravity countercurrently to the upwardly moving gas, and the thus heaty treated fibers are finally discharged from the base of the chimney through' a rotary sealing valve I8 into a carrier stream of cooling air moving through a pipe 20 at substantially atmospheric temperature. The hotgas exits from the top of the shaft through offtakes 2| and at least part of this gas is recycled as carrier gas for transporting more fiber to the topof the shaft. It has been found possible to effect substantial improvement in the freeness or filterability of the fibers without serious reduction in water of crystallization and tensile strength, when the fibers are in contact with the hot gases in shaft I6 .at maximum gas temperatures between 900.-15004F., for periods ranging between fiveV and thirty seconds. During such short period of contact of the fibers with the hot gases at such temperatures (as measured by thermocouples) the temperature of the beris raised to-abeut 600P;41-,200f'F.,-asfdetermined by calorim eter 'Withfthe temperaturefofbtheV gases-#entering-th #shaft from acombustion chamber'iz'hat 1-2`00f=150011'.,iand awithfithe temperature of the bers leaving the base of therchimneyfatabout 900-1200 F., satisfactory improvement Tof the freeness of thef-ber f-is effected when the time of i" heat ftreatmentiwithinthe shaftfii's as ffshort asili-ve seconds. `ifUnder 'these'.conditions'theifreee ness lis iirnproved'f byI .as fmuchtas i90-.4.100 A:with `not .toefexceed i15.% :floss off; molecular '.Watenfand notitobexceedvZO .fdrcpfin.;bertensileastrength.

The degree of improvement of lterabi'lityrof the .lchrysotile bers :iis y directly iproportional: to the temperature and '2; une of heat ztreatment. {Bhestrengthiofxthesbersfistadversely effected: by I-prolqnge`d..heatingat 'temperatures iat @Which-the .filtering characteristics `:are .substantially gim.- rproved. ..1Consequently r.there are optimumfranges of temperature and heating :times1.which.develop satisfactory improvement in :the iilteringY `characteristics Qf.;the iibers withouteriousadverse :eiectvonthe strength; andpfelting characteristics. `,Ampptimum improvement .i in :lterability: of as much. asf 9.0;-1f00`% fcan be effectedswithoutzreduciing :the reinforcing .strength. andzfe'lting chanacfteristics .of the bers` by mote-.thanf25 ibylfheatfing the'fbers "at actual liber-:temperaturesaveraging 700-900 F. for periods ,of timezwhichziare `so;,short (lessthan three.. minutes) was to prevenf .a :loss `.of .combined-water Iiniexcess fof120 `When .usinghot gases @as fthe 4vheating .mediuin, igoodashheating results :are `obtained .whenzthe temperatureof lthe hotgas which :contacts :the ber .gis..abo11t;2001 to 30.0; thigh-er .than ythe .desired maximum temperature; torwhich-the `ber .is vcheated. p hereas `highfrequency electro magnetic. heating :the fibers v,may beheate'd ,to a temperature lof. say 900 '-by.aa.heater whi'chgis .fsetzvto develop.; this same "temperature, .a fhot gas contact flash heater is advantageously Operated with hotgases:averaging"1900f12 00 1F.;.when,it is :desired to .develop actual .:nber temperatures fof `'H10-Pf900 Hnderfzthese conditions ...the :heating .ofthe fibers Y.proceeds atnazsatisfactory rate, .and `it ispossible .to limit the total.. period .of heating w1; inithe :range :live: seconds rtoffthirtyseconds, during dvhh .1710.8 llber temperature is raised rOmxQQInLemperatur-e pr from a preheat temperature not exceeding about 400 F., to the indicated maximumztemperature.

The following voperations were vcarried outin a ash'heating'shaft furnacel having an overall height of Vv2,6 It. and `an lllelilal vdiameter of 'l ft. Hotcombustiongases'entere/the stack from combustion chamber 22 at one side thereof at a nQnt-abpu-t-it. from the base .of the stack. The e ectiveheating length of thev stack is, therefore, out 20 zflt 'lfhecombustiongases rise within stack .at a .velocity .of about 1 ft. perJsecond. mi nbersfwere ied t9 the tonof'the stack conilQusly :at .feed .rates bf 1-,2 :tons per .hour` erjmoceuples ..24 are disposed at intervals :t Oushgutthe .length of the stack; and .icalfarimetermeasurements were made at :frequent iptrvalsngf the .actual .temperatures .of :bers "wthdrawnxfrgm the base Qfthestack.

When operating with a maximum gas temperature of about 950 F. at a point in the stack of about 1-2 feet above the level at which combustion gases enter the stack, and feeding fibers to the stack at the rate of two tons per hour, the fibers were in contact with the hot gases Within the stack for a period of about ve seconds, and were heated to a maximum fiber temperature of CTI Thetensile strengths yofv-someof theheatetreated bers werefdetermined by actual strength measuifements Ein vcompari-son with/tensile strengths of equivalent fibers before heat treatment. #Comlparative 'tensile strength and liiberceltingeproperty V`determinations wereA`4` also made -by-'making -up 'sampleisheets of paper lincorporating the heat-treated v-bers, 'and 'testing saidl sheets nAfor strength in comparison Withfsimilansheets made up lwith non-heat-treated ehrysotile `ofrthe saine 1 grade.

A5 illustrating the eiect of heat treatment pon the speed-'ofmanufacture of-Water#laid-:asbestoscement sheets and on theproperties of; sheets so formed, ta body of chrysotile asbestos fibers of l'suitable shortgber .grade was divided 'into "two batches. Onebatch ofthefibers Washeattreated `ina 'bedrfonaperiod of 'aboutthree minutes Joy contact with heating gas .at 4a "temperature u.of 1200`F. )The rbatch nof thus heat treated .bers

-was incorporated with `finely Vdivided".Poi'ttland Acementjin a conventional asbestos-cementaqueous slurry. containing cement `vand asbestos. 'fibers in '.vapproximately 'theratio of .3:1 by Weight. vThe .batch of untreated 'fibers was likewisemade up in`t o'.a.,s' lurry of similar composition. jrljhe s 1urries.so"formed were-.allowedtoilow antonivtering'loeds of a sheet forming press, and the ram of the .press was .actuated vto .eXpressjfrQm-:the

yiiloer jsheets 4thus y@formed a maj or proportion tof .the water by ltration 4thr-.Ouglrl the Afielted fibers .',afjsupporting screerggunder vrelatively ow initially applied pressure. :Comparison ofthe ini.- tial dewatering rate of the lsheet incorporating 60 Aheat treated asbestosibersfwith the rdewaterling rate of the correspondinggsheet'.made AWith-'the .same .relative proportion gf untreated betsyby conditions :otherwise 'the same, demonstrated 4an increase .ofzabout 75% in `:the initial deW-atering rate and -lter-abili-ty -of the 'sheet formed heat treated fibers. l vSheets thus formed were :compressed under iwith pressures of the order of 2000 lbs. to the square inch, and the dense 4products thus formed were then cured to develop a hardening set. The resulting cured sheets were then subjected to transverse strength tests, and such tests demonstrated that the strengths of the sheets incorporating the heat treated asbestos fibers 4were at least 75-85'% of the strengths of the corresponding 7 sheets embodying conventional non-heat-treated fibers.

Y Filtration rate tests were also made by sheet-- ing out asbestos fibers from an aqueous furnish on a screen lter employing a light vacuum as a means of developing a pre-determined low pressure differential on opposite faces of the Wet asbestos fiber sheet. Comparative filtration rate measurements thus made demonstrated that a filter sheet made from a unit weight of bers heat-treated by contact with hot gas at 1200 F. for a period of about three minutes allows a unit weigh-t of water to filter therethrough at a rate which is 75 %95% faster than the rate at which the same amount of water filters through a sheet of conventional non-heat-treated fibers. Another good way of determining comparative filterability is by measuring the pressures required to force a definite volume of water through a filter sheet of unit fiber content a-t a constant rate. The improvement in filterability of a sheetproduct incorporating the heat-treated fibers is marked even when such sheet contains binder materials such as Portland cement or starch, or when the sheet contains comparatively high proportions of wood fibers or non-heat-trea-ted chrysotile fibers admixed with -75% by weight of heat treated chrysotile fibers.

Canadian #l crude chrysotile asbestos liber has an average tensile strength of about 130,000 lbs/sq. in. After one of these fibers had been exposed for three minutes to a temperature of 800 F. in a high frequency electro-magnetic heating field, and then rapidly cooled, its tensile strength was found to have dropped by about 20% and its cross-Sectional areaV had increased somewhat. The actual increase in the cross-section of the fiber which results from a short period of heat treating is accompanied by a substantial increase in the total surface area of the fiber. The heat treatment yields a fiber which is more open and of increased freeness or filterability, and which has greater absorptive power and lower density. The heat treatment effects some release of molecular water, which causes a surface rupture of the individual fibriles, and these fibriles increase the surface area and impart a roughened surface to the fiber. Although this increase in surface area is small for any single fiber, it is magnified greatly in a given fiber mass. Therefore, the resulting fiber mass is springy and releases water rapidly during the wet filtration operation.

The effect of heat when applied to chrysotile asbestos is to first reduce uncombined surface moisture and to thereafter release chemically bound water of crystallization. If the temperatures and time of heating are excessive, the fiber is completely dehydrated and the molecules rupture to form magnesium silicate and silica. It is important for the objectives of the present invention to limitthe heat treatment to temperature and time limits within which satisfactory improvement of freeness is obtained with not to exceed 20-25% loss of molecular water and tensile strength. Y

The invention which has been thus described by `detailed example is vnot limited as `to such details and it is to be understood that variations, changes and modifications are contemplated within the scope of the` invention as defined by the following claims.

What we claim is:

1. The method of treating chrysotile asbestos fibers to improve the filtering properties thereof which comprises, heating the fibers up to a temperature of 60o-1200" F., maintaining the fibers at such temperature for a period of between five seconds and three minutes, and cooling the thus heated fibers. 1 y

2. The method of treating chrysotile asbestos fibers to improve the filtering properties thereof which comprises, heating the individual fibers to a temperature within the range GOO-900 F., limiting'the period of heating of the fibers at such temperatures to about threeminutes, and cooling the thus treated fibers.

3. A process of heattreating chrysotile asbestos fibers to improve the filtering properties thereof which comprises, forming a suspension of the fibers in a flowing stream of hot gas at gas temperatures ranging from 900-1500 F., removing the fibers from the Vhot gas stream after a period of contact with the gas of between five seconds and 1/2 minute, and rapidly cooling the thus heatedfibers. I Y

. 4. A process of treating chrysotile asbestos fibers to enhance the filtering properties thereof which comprises, heating the individual fibers by contact with a fiowing stream of hot dry gases at gas temperatures in the range 900-1500 F., limiting the time of exposure of a fiber to such temperature within a period of five seconds to 1/ minute, and rapidly coolingthe thus treated fibers.

5. Heat treated chrysotile asbestos fibers having the properties obtained by heating chrysotile fibers in accordance with the process of claim 1.

6. Heat treated chrysotile asbestos fibers having the properties obtained by heating chrysotile fibers in accordance with the process of claim 3.

7. Heat treated chrysotile asbestos fibers having the properties obtained by heating chrysotile fibers in accordance with the process of claim 4.

MARION S. BADOLLET. WILLIAM C. STREIB.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 517,852 Ehrlicher Apr. l0, 1894 1,430,085 Keeth Sept. 26, 1922 1,455,975 Spencer May 22, 1923 1,887,726 Weber Nov. 15, 1932 1,894,250 Wilson Jan. 10, 1933 1,973,407 Cowles Sept. 11, 1934 2,029,524 Denning Feb. 4, 1936 2,108,577 Brough Feb. 15, 1938 2,385,384 Schroy Sept. 25, 1945 2,413,134 Barrer Dec. 24, 1946 2,445,415 AndersonV July 20, 1948 

1. THE METHOD OF TREATING CHRYSOTILE ASBESTOS FIBERS TO IMPROVE THE FILTERING PROPERTIES THEROF WHICH COMPRISES, HEATING THE FIBERS U TO A TEMPERATURE OF 600-1200* F., MAINTAINING THE FIBERS AT SUCH TEMPERATURE FOR A PERIOD OF BETWEEN FIVE SECONDS AND THREE MINUTES, AND COOLING THE THUS HEATED FIBERS. 